Geological characterizations, modeling and 3-D computer-generated visualizations of the Ordovician St. Peter Sandstone at the Hillsboro Gas Storage field in Montgomery County, Illinois, are discussed. Petrophysical analyses reveal four distinct hydraulic flow units in six cored wells. Furthermore, four lithologies, identified by thin section petrography, are associated with the various hydraulic units. Fieldwide visualizations of 3-D distributions of petrophysically-derived attributes – reservoir quality index (RQI) and flow zone indicator (FZI) – show considerable vertical variability but lateral continuity. This finding explains why it is easier to expand the gas bubble laterally than vertically. Advantages of the 3-D reservoir description of Hillsboro Gas Storage field include 1) improved definition of the spatial porosity distribution which leads to better estimation of reservoir volumetries; 2) improved definition of reservoir hydraulic flow zones; and 3) development of realistic reservoir model(s) for the simulation and management of the gas storage field.